A) Field of the Invention
The present invention relates to a semiconductor device and its manufacture method, and more particularly to a semiconductor device using nitride semiconductor such as gallium nitride (GaN). The present invention is applicable to a nitride semiconductor device, particularly a high voltage breakdown nitride semiconductor device.
B) Description of the Related Art
Some transistors used in a base station of mobile phones and for satellite communications are required to realize high temperature operations, high speed switching and high power operations. Nitride semiconductor, typically gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN) and mixed crystal thereof have drawn attention as the material of high output power devices and short wavelength optical emission devices because of their excellent characteristics. Many reports have been made on field effect transistors, particularly high electron mobility transistors (HEMT's), as high output power devices. Typical HEMT's under researches are HEMT using a GaN layer as a channel layer and an n-type aluminum gallium nitride (AlGaN) layer as a carrier (electron) supply layer.
JP-A-2004-186679 teaches a method including processes of: depositing an amorphous GaN layer on a c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD); forming a crystalline GaN buffer layer by raising a temperature to 950° C.-1050° C.; forming GaN growth nuclei on the crystalline GaN buffer layer; forming a GaN channel layer having a thickness of 100 nm or thicker, an Si doped n-type AlGaN carrier supply layer having a thickness of 10 nm-20 nm and a Si doped n-type GaN contact layer having a thickness of 10 nm-20 nm by MOCVD; forming source/drain electrodes; then etching the n-type GaN contact layer in a gate electrode forming area; conducting an annealing process in a nitrogen atmosphere; and forming a gate electrode. It also teaches that if a silicon carbide (SiC) substrate is to be used, it is preferable to form the buffer layer of AlN.
International Publication WO2004-066393 teaches forming a GaN device layer on an Si substrate, via a buffer layer made of a lamination of a plurality of Al (In, B) GaN layers (e.g., alternate lamination of AlN layer, p-type GaN layer and GaN layer), and also teaches covering the surface of the device layer with a silicon oxide film, patterning the silicon oxide film to form an opening, and forming an electrode in the opening.